Dupont epoxy black. Goes on with no need to primer. Just sandblast the frame and spray the epoxy black on it and call it done.
Or if you have a little cash to spend, take it somewhere and get it powder coated.
Another option would be to galvanize it if you can find a plant close by with large enough tank to fit a frame in.
I've never dealt with POR15 and probably won't. My opinion is, like powder coating, it will be hard, if not impossible, to remove if the need ever arises that you need the coating off to work/fix/repair the frame. Thats why I say use epoxy paint as you can repaint over it and epoxy is real tough and durable paint. /forums/images/graemlins/waytogo.gif